Commit c80ee952a1 removed temporary store used
to contain changes of the block being processed. It's wrong in that the block
changes should be applied to the database in a single transaction so that
there wouldn't be any intermediate state observed from the outside (which is
possible now). Also, this made changes commiting persist them to the
underlying store effectively making our persist loop a no-op (and not
producing `persist completed` log lines that we love so much).
